1. Field of the Invention:
The present application relates to analog to digital conversion and more particularly, to an analog to digital converter, which though not limited to, finds particular application in a pulse height analyzer.
2. Description of the Prior Art:
In the field of spectroscopy as well as in other scientific studies produced pulses which represent various phenomena such as energy levels are analyzed to learn about the phenomena represented by the pulses. For example, a high spectral resolution spectrometer produces many pulses of different heights. These are typically analyzed in a multichannel pulse height analyzer, wherein the pulses are digitized as a function of their detected peaks. Each channel is associated with a different digital output. It effectively stores a count or number, indicating the number of pulses which were digitized during the period and produced the particular digital output associated with the channel. In such an application, it is of prime importance to include an analog to digital converter (ADC), whose digital outputs when counted in the different channels results in equal width channels. Variations in channel widths with respect to the average width is generally referred to as differential linearity. Differential linearity is very important since differences in channel widths can cause the appearance of false peaks and mislocation of true peaks in mislocated channels.
Differential linearity is related to the desired digital resolution. In a successive approximation type ADC in which an N bit resistive ladder is included to digitize the pulses into N-bit numbers, the % differential linearity is equal to the % resistor matching tolerance in the ladder times 2.sup.N. For 13-bit resolution, i.e., N=13. Even with a resistance matching tolerance of 0.005%, the % differential linearity is about 41%. Such high differential linearity is too high for many accurate studies.
Another important property of an ADC in a pulse height analyzer is stability due to the relatively long period during which data, i.e., pulses are received. Also good integral linearity is important, in order to be able to relate the phenomena, e.g., energy represented by the digitized peaks to the energy represented by calibrated peaks.
Some pulse height analyzers with ADC's of up to 13 bits, are at present, available commercially. They are represented as having very good differential linearity. However, their stability is not sufficiently high for certain applications. Also, they do not seem to be easily modifiable to provide resolution beyond 13 bits. Thus, a need exists for a new ADC which exhibits the above described properties, including good differential linearity.